This invention relates to an air-fuel ratio feedback control system for performing by electronic means feedback control of the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine, and more particularly to an air-fuel ratio feedback control system of this kind, which is capable of initiating positive control of the air-fuel ratio to proper values immediately upon starting of the engine.
A fuel supply control system for use with an internal combustion engine, particularly a gasoline engine, has been proposed e.g. by U.S. Ser. No. 348,648, now U.S. Pat. No. 4,445,483, assigned to the assignee of the present application, which is adapted to determine the valve opening period of a fuel quantity metering or injection means for control of the fuel injection quantity, i.e. the air-fuel ratio of an air-fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
According to this proposed system, feedback control of the air-fuel ratio is carried out when the engine is operating in a normal operating condition, wherein the valve opening period of the fuel injection means is controlled by varying the value of a coefficient in response to the output of an exhaust gas concentration sensor arranged in the exhaust system of the engine, whereas open loop control of the air-fuel ratio is carried out when the engine is operating in particular operating conditions such as idling region, mixture leaning region, wide-open-throttle region, and decelerating region, wherein are applied coefficients which have predetermined values appropriate to respective ones of the particular operating conditions so as to achieve respective optimum air-fuel ratios. Thus, the proposed system can achieve improved characteristics in respect of fuel consumption and driveability.
It is thus desirable that the predetermined air-fuel ratios corresponding to the respective particular operating conditions can be attained without fail by means of open loop control. However, as a matter of fact, the actual air-fuel ratio can sometimes have a value different from a desired predetermined value due to variations in the performance of various sensors for detecting the operating conditions of the engine and a system for controlling or driving the fuel injection means. This makes it difficult to ensure required operational stability and driveability of the engine.
To avoid such disadvantage, it has been proposed by U.S. Ser. No. 376,106 assigned to the assignee of the present application to calculate and store as a second coefficient a mean value of values of a first coefficient applied during the air-fuel ratio feedback control responsive to detected values of the exhaust gas concentration, and apply the second coefficient or the mean value during subsequent open loop control, thus controlling the air-fuel ratio to values closer to the predetermined values appropriate to the respective particular operating conditions of the engine during the open loop control.
However, if the stored value of the second coefficient is erased upon interruption of the operation of the engine, the second coefficient applied after the restart of the engine will not have an appropriate value until after the feedback control has been carried out for a substantial period of time. As a consequence, the engine suffers from low operational stability and degraded driveability before the lapse of the above substantial period of time.